Early sensor technology for infrared thermal imaging applications typically used mechanical scanning systems to focus infrared energy onto a single element detector. As a result, the displayed thermal images often had poor resolution, and visible light photographs were often required to identify the object of interest in a thermogram. Early infrared sensors, such as that depicted in U.S. Pat. No. 4,507,551 (Howard et al.), also relied on liquid nitrogen or compressed gas in order to cool the sensor, a requirement which was later eliminated by the introduction of Stirling cycle and thermoelectric coolers.
Many current infrared imagers now use focal plane array (FPA) detectors. These multi-element, solid-state detectors are arrayed together to provide a high-resolution image and to eliminate the need for a mechanical scanning system within the optical path. The introduction of FPA imagers has dramatically improved infrared imaging by providing high-resolution imaging devices with greatly reduced size and weight. Typically, FPA detectors have more than 70,000 elements or pixels. As a result, thermograms taken with an FPA imager often no longer require a corresponding visible light control photograph to help identify objects of interest.
The FPA imaging devices that are currently available may have detectors that are either cooled or uncooled. Cooled FPA imaging devices, which have been commercially available since the early 1990s, operate in the 3–5 μm and 8–14 μm range and generally provide excellent sensitivity. Uncooled FPA imaging devices are a more recent introduction. Unlike previous infrared systems that directly sensed photons, these later systems, which may be adapted to operate in either the 3–5 μm range or the 8–14 μm range, function by sensing changes in electrical resistance across the detector due to temperature changes.
The advent of uncooled FPA detectors for the 3–5 μm and 8–14 μm regions of the infrared has drastically reduced the price, weight, and complexity of infrared imaging devices for these regions. The remaining high-cost and heavy item in these systems is the germanium, zinc selenide, or chalcogenide glass lens, which was formerly used with the expensive and complex cooled detector imaging devices and which is still used with their uncooled replacements. These lenses cannot be made by molding, but instead must typically be diamond turned or ground and polished one at a time. (Chalcogenide glasses can be molded, but the molded blanks typically require postprocessing to be usable.) Moreover, because of their high indices of refraction, it is typically necessary to use antireflection coatings with these lenses, which further adds to their cost and manufacturing complexity. Moreover, even with the very high indices of refraction that germanium and zinc selenide lenses afford, many 3–5 μm and 8–14 μm thermal imaging devices based on these materials still require more than one lens element in order to cover the desired angular field of view with the required image spot size or modulation transfer function (MTF).
In addition to the heavy germanium, zinc selenide and chalcogenide glass lenses noted above which have been typically used in FPA imaging devices, some lighter Fresnel lenses are also currently known to the optical arts, including some which are made out of plastics that transmit in the 8–14 μm region of the infrared. For example, infrared-transmitting plastic Fresnel lenses are currently available commercially from Fresnel Technologies, Inc., Fort Worth, Tex. Some of these lenses are described in commonly assigned Reissued Pat. 35,534 (Claytor). Infrared-transmitting plastic Fresnel lenses are also described in I. Pasco, “Infrared Motion Detection Optics in Plastics: Design, Manufacture, and Application”, OSA Proceedings of the International Optical Design Conference, Vol. 22 (G. Forbes Ed, 1994). Those lenses, which are generated from a diamond turned master surface through injection molding or compression molding of high density polyethylene, are designed to operate in the 8–14 μm region of the infrared.
Fresnel lenses are advantageous in that they offer a substantial reduction in bulk lens thickness in comparison to regular aspheric lenses. This result can be understood with reference to FIG. 1, which depicts a conventional Fresnel lens 11 that might be used when a reduction is thickness or weight is desired. The Fresnel lens depicted therein comprises a periodic refractive structure of concentric prisms 13. The surfaces of these prisms are designed to refract light by collapsing the aspheric surface 15 of a corresponding conventional lens nearly into a plane. Hence, the reduction in bulk lens thickness is essentially equal to the volume bounded by the original aspheric surface and the new lens surface defined by the surfaces of the prisms. This bulk reduction allows Fresnel lenses to be substantially thinner than their conventional aspheric counterparts.
The refractive surfaces of the prisms making up the Fresnel surface may be referred to as grooves 17 and drafts 19. The grooves are the actual surfaces that are used to approximate the continuous curvature of the aspheric surface of a conventional lens, while the drafts are the necessary discontinuities between the grooves that are required to return the curvature of the lens back to a plane. The lateral distance between the peaks of adjacent grooves is referred to as the pitch 21. The more common type of Fresnel lens for use in the 8–14 μm region of the infrared is described in commonly-assigned patent Re35,534. This lens does not have a constant pitch; instead, the depth of grooves 17 is held constant, so that the groove width (also known as the pitch) varies. The surface of each groove 17 is also individually manufactured to an aspheric contour, rather than the frusto-conical approximation typical of the style of Fresnel lens illustrated in FIG. 1.
While it would be desirable to substitute plastic lenses for the much heavier and more expensive germanium, zinc selenide and chalcogenide glass lenses commonly used in thermal imaging devices, such a substitution is beset by many difficulties. The use of non-Fresnel plastic lenses for this purpose is generally not feasible, due to the poor transmission properties that such lenses have in this region of the spectrum. For example, high density polyethylene, the plastic most commonly used for infrared transmitting lenses in the 8–14 μm region, has poor transmittance characteristics compared to other, more familiar optical materials.
Furthermore, the plastics most commonly used in the 3–5 μm region, polychlorotrifluoroethylene (PCTE) and polytetrafluoroethylene (PTFE), have very low indices of refraction, in addition to poor transmittance at wavelengths longer than 4 μm. While polyethylene has a somewhat higher index of refraction in both the 3–5 μm and the 8–14 μm regions of the infrared, it is still quite low when compared to conventional infrared lens materials such as zinc selenide and germanium. It is therefore not practical to use ordinary lenses based on these plastics for thermal imaging applications, because the large thickness of an ordinary lens with a low f/number (or made from a very low index material) would make the ordinary lens impractically opaque (or at least impractically thick) at its center.
While the use of a plastic Fresnel lens in thermal imaging devices would avoid some of the problems encountered with normal plastic lenses, the use of such a lens is beset by its own difficulties. For example, when typical single element plano-convex plastic Fresnel lenses are incorporated into thermal imaging devices, the image quality of the device suffers markedly. In particular, while image quality is found to be adequate on axis, it is found to deteriorate to the point of uselessness only a degree or so off axis.
In some visible light applications, as in rear projection devices, two Fresnel lens elements (a Fresnel condensor lens and a Fresnel field lens) are used in combination, typically with an LCD panel disposed between them. A description of such a system may be found in A. Davis et al., “P-95: Fresnel Lenses in Rear Projection Display Systems”, SID 01 Digest (pp. 1–5). However, it has not generally been considered practical to make infrared imaging devices with multi-element plastic Fresnel lenses, both because the typical loss through each element in this region of the spectrum is relatively large compared to germanium or zinc selenide lenses, and because of the previously noted problems with off-axis performance characteristics. Recent advances in uncooled focal plane arrays have further exacerbated these issues. In particular, the arrays have become larger in size with smaller sensitive areas (“pixels”), thus increasing the angular field of view requirement while decreasing the required spot size.
There is thus a need in the art for a lightweight, inexpensive lens which is suitable for use in thermal imaging applications, and which has acceptable off-axis performance characteristics and image quality. There is also a need in the art for imaging devices that are based on such a lens. These and other needs are met by the devices and methodologies disclosed herein.